1. Field of the Invention
The present invention relates to a method of fabricating polysilicon films.
2. Description of the Prior Art
Currently, liquid crystal displays (LCDs) are the most popular flat panel display technology. The applications for liquid crystal displays are extensive, such as mobile phones, digital cameras, video cameras, notebooks, and monitors. Due to high quality display requirements and the expansion of new application fields, the LCDs has developed toward a direction of high quality, high resolution, high brightness, and low price. Development of low temperature polysilicon thin film transistors (LTPS TFTs), to be used in active matrix LCD, is a break-through in achieving the above objectives.
With reference to FIG. 1, FIG. 1 is a perspective diagram showing the means of fabricating a low temperature polysilicon film according to the prior art. As shown in FIG. 1, a substrate 10 is provided, in which the substrate 10 is composed of transparent material such as glass. Next, a barrier layer 12 composed of silicon oxide (SiO2) or silicon nitride (SiN) is disposed on the substrate 10, in which the barrier layer 12 is amorphous in nature. Next, an amorphous silicon (a-Si) layer 14 is placed on the barrier layer 12, and a laser annealing process 16 is performed to utilize an excimer laser to irradiate the amorphous silicon layer 14 and cause a crystallization of the amorphous silicon layer 14.
During crystallization, the amorphous silicon layer 14 will undergo a melting and re-growth transition and transform into a polysilicon layer. The grain size of the resulting polysilicon is dependent upon the energy of the laser annealing process, such that the polysilicon grain size increases with laser energy density up to an energy level lower than that required to completely melt the silicon layer, an energy level referred to as “full-melt threshold”. After the laser energy reaches the full-melt threshold or surpasses the full-melt threshold, the grain size of the polysilicon layer will become small again, resulting in microcrystalline forms. Essentially, this phenomenon is caused by the fact that no nuclei will survive at the full-melt threshold as the amorphous silicon layer 14 grows on the amorphous barrier layer 12, and during the cooling stage, many nuclei will be formed resulting into many small grains. Hence, in order to maximize the size and uniformity of the grains, the optimum laser energy value for the crystallization process should be maintained just below the full-melt threshold value. Typically, the optimum laser energy range, also referred to as the “process window”, is of the order of 10 mJ/cm2. Ultimately, the small process window of the conventional method leads to non-uniformity and non-reproducibility of silicon crystal quality due to spatial and run-to-run variations of laser beam energy, which may eventually be larger than the process window.
Additionally, the full-melt threshold is dependent upon the thickness of the silicon layer. Hence, spatial and run-to-run variation in silicon thickness will also result in non-uniformity of the crystal quality. Another disadvantage of the prior art is that each region of the amorphous silicon layer must be irradiated with large number of pulses (often referred to as the number of shots) to obtain uniform grain size. In most cases, a region is irradiated 20 to 40 times before achieving satisfactory and uniform grain size and the frequent irradiation performed will ultimately reduces the throughput of the laser annealing process.